1. Field of the Invention
The present invention relates to a scanning device, and more particularly, to a scanning device capable of reducing a warm-up time.
2. Description of the Prior Art
Scanning devices, such as fax machines, scanners, copiers, and multi-function products with integrated support of print, scan and fax functions, have become popular electronics devices. These devices have a scanning device for obtaining a corresponding image from a document.
Please refer to FIG. 1, which is a block diagram of a conventional scanning device 10 according to the prior art. The scanning device 10 comprises a cover 11, a housing 12, a cold cathode fluorescent lamp (CCFL) 14, a photosensor 16 and a controller 18. The CCFL 14 is used for producing light. The photosensor 16, which can be a Charge-Coupled Device (CCD), is used for detecting the light produced by the CCFL 14 that is reflected by a document 15. The controller 18 is used for controlling operations of the scanning device 10.
The basics of the scanning device 10 are described as follows. First of all, the desired document 15 is placed onto the scanning device 10, and then the cover 11 is closed to prevent ambient light from interfering with the subsequent scanning procedure. The CCFL 14 generates light to illuminate the document 15, and the photosensor 16 detects the reflected light from the document 15. Since less light is capable of being reflected from dark areas of the document 15, while more light is reflected from the light areas of the document 15, the photosensor 16 can detect intensity variations with respect to the reflected light. Finally, the controller 18 transforms the intensity of the reflected light into digital data, and combines these digital data into an image file.
Please refer to FIG. 2 in conjunction to FIG. 1. FIG. 2 illustrates a relationship between intensity variations of the cold cathode fluorescent lamp versus time during a warm-up time period. Color image information is obtained by using the CCFL 14 of the scanning device 10 to scan a single time, and this is possible when a sufficiently stable and bright light is produced by the CCFL 14. To do so, the CCFL 14 must reach a sufficiently high temperature to excite enough ions to strike the fluorescent material disposed on the glass tube of the CCFL 14. In general, the period required to heat the CCFL 14 to the sufficiently high temperature is called the warm-up time period. A warm-up time period (t0˜t1) of 45 to 90 seconds is typically necessary. During the warm-up time period, a substantial luminance variation of the light provided by the CCFL 14 is exhibited. If the luminance variation is too great, corresponding errors for the reflected light detected by the photosensor 16 are created. From FIG. 2, the luminance of the light provided by the CCFL 14 in the period of t0-t1 is too great to permit color scans. As an example, during the period of ta˜tb as illustrated in FIG. 2, when using the CCFL 14 to scan the area 17 of the document 15 having a uniform gray level, ideally the photosensor 16 should detect the same reflected light luminance. However, because the luminance of the light provided by the CCFL 14 at the time tb is larger than that at the time ta, the photosensor 16 detects a different luminance for the same gray-level area 17, causing the controller 18 to read error-laden data, which leads to incorrect image data. Hence, the CCFL 14 of the conventional scanning device 10 requires a relatively long warm-up time for generating a sufficiently stable and bright light to ensure a high quality scan.
To reduce the warm-up time period, U.S. Pat. No. 6,316,767 to Paxton et al., which is incorporated herein by reference, discloses a scanning device having a photosensor for detecting ambient light to control the on-and-off state of a light source so as to shorten a wait time for the light source to warm-up. For instance, when a cover of the scanning device is opened and a document is put onto the scanning device, the photosensor detects the ambient light, and then the CCFL begins to warm-up. In this way, when the cover is closed and a scan command is executed, the CCFL has already begun to warm up so that the user wait time is reduced. Strictly speaking, the start time of the warm-up time period of the CCFL is advanced rather than the warm-up time period being shortened. Therefore, if the CCFL requires a lengthy heating time, even if the warm-up time period is advanced, a substantial user wait time for the CCFL warm-up is still necessary.